1. Field of the Invention
The present invention generally relates to the field of changing the optical characteristics of an energy focusing system and more specifically relates to changing the focal length of an imaging mirror as employed in a solar collector field.
2. Description of the Prior Art
Solar collection systems are generally constructed so as to receive radiant energy from the sun and convert it to heat or electrical energy. In each system, it is an object thereof to obtain the greatest concentration of energy using the minimum number and least costly elements. In many installations, where great intensity of solar energy is required, focal elements such as lenses or mirrors are employed to concentrate the solar radiation onto a receiver.
Spherical mirrors are chosen in some solar collector installations for their individual ability to receive radiation from an off-axis source and reflectively focus that radiation to a relatively small area. However, spherical mirrors are unable to focus an image of an off-axis point source of radiation at infinity to a single point because of an inherent astigmatism. Therefore, the optimum results one can expect from a spherical mirror is to focus the off-axis radiation to a relatively small and stably oriented area. The smallest stably oriented area is termed the "circle of least confusion" and is shown in FIG. 1 as being the optimum focus of an off-axis point source at infinity occuring in a plane perpendicular to the axis of a focused bundle of light rays.
In FIG. 1, a spherical mirror 10 is shown having a central axis 12, along which a center of curvature C and a focal point P lie. The distance from the center of curvature C to the point A where the central axis 12 intersects the spherical mirror 10 is defined as the radius of curvature for the spherical mirror 10. The focal length of the mirror is related to the radius of curvature as one-half the radius of curvature. Therefore, PA=CA/2 for radiation received along a path parallel to the central axis 12.
Two orthogonal planes of reflection are shown in FIG. 2, which extend from the reflective surface of the spherical mirror 10 to illustrate the problem of astigmatic aberration of the focused image when the spherical mirror 10 is oriented so that the incident solar radiation is at an angle .theta. with respect to the central axis 12. A first plane extending from the dashed line R, S represents how rays of radiation impinging along line R, S are focused at a point T along the line 14 extending from point A. Line 14 represents the fixed direction to which the focused bundle of rays, from the mirror 10 to a fixed target, are maintained during track with a corresponding change in .theta.. A second plane extending from dashed line J, K perpendicular to and intersecting line R, S at point A, represents how rays of radiation incident along line J, K are focused at a point L on the line 14 extending from point A. As is evident from the FIG. 1 illustration, the astigmatism of the spherical mirror causes a first line focus of energy to be made intersecting point T and a second line focus of energy to be made, orthogonal to the first line of focus, intersecting point L. Between the first and second lines of focused energy, along a plane mutually perpendicular to the aforementioned first and second orthogonal planes extending from the mirror 10, the energy is focused in a circle E which is defined as the "circle of least confusion".
It has been found that although the circle of least confusion is the most ideal concentration of energy for a spherical mirror, as the angle .theta. changes, the distance along line 14 from A, to E changes.
Until now, such a change in the distance was either tolerated as an inherent deficiency in thermal transfer or compensated for by using additional mirrors and their associated heliostat mounting mechanisms to increase the amount of collected energy.